Generally, for that a resolution of a display panel of a current portable display device (e.g., a notebook) is constant, a graphic card is designed as directly outputting an image data with a constant resolution to the display panel without performing any image scaling or image processing during the transmission. In other words, a user is only allowed with performing image processing via a built-in VGA card to achieve an equivalent effect in adjusting characteristics of the display panel of the portable display device. However, as complexity of image data grows and a display panel becomes more and more mature day by day, the foregoing adjusting approach gradually becomes inadequate in meeting user (observer) requirements. Therefore, it is necessary to provide a display mechanism utilized in a portable display device to meet user requirements.